Since an LED has many advantages including small volume, short response time, low power consumption, high reliability, and high mass-production feasibility, the LED is widely applied as a light source in various electronic devices. For example, the LED serves as a backlight source of a liquid crystal display (LCD) to replace a conventional fluorescent tube.
FIG. 1A shows a conventional LED driving system mainly comprising a plurality of LED strings 10, a minimum voltage selector 12, a boost controller 14, and a boost power stage circuit 14.
For the conventional LED driving system illustrated in FIG. 1, although every LED string 10 implements the same voltage source VDC and the same number of LEDs 100, the LEDs 100 in each LED string may not match with one another, where voltages at input pads 11 are different. Therefore, in order to reduce power consumption of the LED strings 10, the minimum voltage selector 12 is configured to select a minimum voltage and the boost controller 14 and the boost power stage circuit 16 control a voltage source VDC, so that the voltages at the input pads 11 are regulated at the minimum voltage.
The conventional LED driving system illustrated in FIG. 1A allows the current sources IS to turn on and turn off via a light modulation signal pulse-width modulation (PWM), which simultaneously controls to turn on or turn off the boost controller 14. In addition, when the boost controller 14 is turned off, the boost power stage circuit 16 is turned off; otherwise, a sharp overshoot voltage would occur for the outputted voltage source VDC.
FIG. 1B shows a schematic diagram of waveforms of the light modulation signal PWM and the voltage source VDC. Although overshoot voltage does not occur in the voltage source VDC, when the light modulation signal PWM is at a logical-low level state (or is turned off), VDC undergoes a falling transient that is created due to capacitors and resistors in the system, thereby causing an unstable load current of the boost power stage circuit 16. As a result, currents flowing on the LED strings 10 and voltages at the input pads 11 are unregulated.
Therefore, a novel light modulation mechanism is in need to regulate the LED driving system.